Thursday, August 5, 2010

One of my recent acquisitions for the Discombobulator, as documented in the previous post, is an Encore MFS01 Frequency Shifter, in MOTM format. Encore also offers this in Frac and Euro formats; the MOTM-format unit had been out of stock for some time, but this year Encore has been doing new runs of its MOTM-format modules. I received mine a couple of months ago, but I didn't have a place to install it until I built the new block that I documented in my previous post. So I'm just now getting to play with it.

A Frequency Shifter is Not the Same as a Pitch Shifter

So what's a frequency shifter? Well, I had a bunch of material that I wrote to address all that, but I've decided to save it for a follow-up post. To keep it short, a frequency shifter is sort of like a pitch shifter, but it does not maintain the harmonic or musical relationships between the various tones and sounds that make the input. What's that good for? Well, for one thing, it's great for bell and chime sounds, and it behaves a lot more predictably than a ring modulator in doing that job. It can do flanging-like effects that range from subtle to startling. It can do creation of sounds that play in unusual intervals and scales. But if you want to completely brutalize a sound, rip it apart and then glue the pieces back together like a ransom note, a frequency shifter is what you want.

Why Frequency Shifters are Usually Expensive

The classic frequency shifter is the one developed by Harald Bode in the early '70s and licensed to Moog. They were sold both as modules for Moog modulars, and as stand-alone devices. They were as renown for their sound as they were notorious for their price tag -- they sold for about $1000 in 1975 dollars. Obviously that put them out of reach of most musicians, and so not that many were made. They are quite rare and valuable now.

One of the reasons the Bode frequency shifter was so expensive was the sheer amount of circuitry they contained. There are several hard problems that a frequency shifter design has to address. One of these is the problem of generating two sine-wave carrier signals "in quadrature", that is, identical in frequency but separated in phase by 90 degrees. The Bode design was noted for its ingenious approach to this, which produced a very clean pair of carrier signals, but it was costly to implement.

With the benefit of three decades of subsequent technology development, Encore was able to take a different approach to this problem, using an option that wasn't available to Bode: go digital for the carrier generation portion. Encore incorporated a microprocessor that generates the quadrature sine waves from (I presume) an internal look-up table. This makes it easy to maintain the phase relationship while responding to panel controls and control voltage, and it takes a lot less circuitry than the Bode design. Thanks to this, Encore is able to offer their frequency shifter at a lower cost ($399 USD) than competing models from Modcan (the Modcan 39B sells for about $1050 as I write this) and Club of the Knobs (the COTK 1630 lists for E950, about $1250 at the moment). Note that the audio signal path is still all analog -- only the generation of the carriers is digital.

(In fairness, it should be noted that Modcan has two models, the all-analog 39A/B, and the all-digital 65B, which is a dual unit. The 65B currently list for $770.)

The Panel and Controls

With that, let's take a detailed look at the panel. Here's the top portion:

The large Initial Shift and the smaller Fine Shift knobs together set the amount of frequency shift. The Fine Shift knob has a range of about 150 Hz in either direction. The Initial Shift control has a range of approximately 3500 Hz; on the unit I have, all of the action takes place between, roughly, the -4 and +4 positions of the knob -- the 4-to-5 areas have no effect. The Initial Shift knob has a deadband at the zero mark that is useful in using the Fine Shift knob to achieve small frequency shift settings. However, the deadband is also rather disconcerting because there is a jump of about 100 Hz when the knob is moved off of the deadband. To do shifts of less than 100 Hz, you must center the Initial Shift knob on the deadband and then use the Fine Shift.

The Input Gain control attenuates the input signal to the frequency shifting circuits. Note that said signal consists of a mix of three things: the signal from the input jack, and the signals being fed back to the shifting circuitry by the Up Feedback and Down Feedback controls. When using the feedback, you'll find that you have to turn this down some to avoid overloading the input. The red LED next to the knob indicates clipping. The Frequency CV attenuates the signal being fed into the frequency control voltage jack.

Below is the lower half of the panel:

We'll start with the jacks. The Input jack is obviously the input for the audio signal to be processed. The CV jack accepts a control voltage (range +/- 5V) which controls the amount of frequency shift, along with the Initial Shift and Fine Shift knobs. Note that response to control voltage is linear, at a rather measly 100 Hz/volt with the Frequency CV control on 10, so you can't do huge sweeps with the CV. The Up Out and Down (DN) Out jacks are the two outputs from the shifter. The Down Out output responds to the reverse of the shift controls and the CV; in other words, when the Initial Shift knob is turned to the right, the output at the Up Out jack increases in frequency, but the output at the Down Out jack decreases in frequency. Note that the signal present at both of these jacks is 100% "wet"; there is no provision for mixing in any of the unprocessed dry signal. If you want that, you have to use an external mixer.

The Up Feedback and Down Feedback knobs feed some of the output of the Up Out or Down Out jacks, respectively, back to the input. Be careful with the Down Feedback since it can create a "ping-pong" resonance in the circuit which can result in runaway feedback if the input signal hits a resonant frequency. If this occurs, you have to turn the input trim control down to 0 to clear it, which could be embarrassing in live performance.

The device makes available the two sine-wave carrier signals at the Sine Out and Cos Out jacks. The two knobs control the signal level present at these jacks. These two signals will always be at the same frequency, which is determined by the amount of frequency shift (which means their frequency is also effected by the frequency shift CV). The cosine signal leads the sine signal by 90 degrees when the frequency shift is positive; the opposite is true when the frequency shift is negative. The two lights next to the knobs light when each signal is near its positive peak; at low frequencies, they provide visual indication of both the frequency and the direction of the shift.

Testing and Demonstration

I started off doing some tests with some simple waveforms; here is a clip, which I'll refer to as we go through the description. (The clip is an uncompressed WAV file, to avoid any MP3 artifacts that might be triggered by the unusual timbres.) First, I fed a sine wave into the input and tried various frequency shift settings, starting with the fine shift. The pitch of the sine wave goes up like you would expect it to, within the range of the fine shift control; this is at 0:07-0:25 on the clip. Then, I advance the Initial Shift as far up as it will go. This is at 0:33-0:56.

Next, I move the Initial Shift into the downward range. Going the other way, something interesting happens which is characteristic to frequency shifters. As you turn the knob left of the zero mark, the pitch gets lower and lower, goes through bass and subsonic -- and then starts going up again. What's happening is that the shifter has actually taken the input through zero Hz, and it is now outputting a "negative" frequency. As it happens, a negative frequency sounds the same as the corresponding positive frequency; the output signal is inverted, but you can't hear that in isolation. But you notice the difference as the source goes up and down: when the frequency of the source decreases, the frequency of the output increases, and vice versa! It's called frequency reversal, and it makes sense when you look at the math: if the source is at 400 Hz, and the frequency shifter takes it 1000 Hz in the negative direction, the resulting frequency is -600 Hz (which sounds the same as positive 600 Hz). If you lower the frequency of the input, it's going towards zero and the subtraction is moving further away from zero, taking the absolute value of the output higher. If you lower the input to 100 Hz, now the output is at -900 Hz. This is the much-talked-about through-zero operation, and it's something that conventional pitch shifters can't do. In the clip, it starts at 0:56; it goes to subsonic and starts into the negative frequency range at 1:00. From 1:10 to 1:18, I twiddle the frequency knob on the source VCO, and the frequency coming out of the shifter responds opposite to it; this is frequency reversal. If you do this with a signal containing a mix of tones, the higher tones will be lower than the lower tones in the frequency-reversal region, which can do some truly bizarre things to natural sounds like voices and animal noises.

In the clip, you can hear a subtle change in the timbre of the sine wave as I manipulate the frequency shift. Now, a sine wave should not have its harmonic content effected by the frequency shifter since it theoretically doesn't have any harmonic content; it should just go up and down in pitch, with no noticable change in timbre. That isn't quite what happens: as I went up in frequency, some odd sub-harmonic tones started to appear. They weren't very loud, but they were audible. I'm not sure where they are coming from. These also appeared as overtones when I went into the negative frequency range. Perhaps the module is picking up electrical noise from something else in the room. Another possibility is that the module's carrier suppression is not quite perfect, and the carrier or some overtone of it is leaking into the output. It could also be that the sine-wave output on the VCO that I used as the source (a Dotcom Q106) is not absolutely pure; that would not be unexpected, since producing pure sines is a very difficult thing for a VCO to do. (I did also try the fixed 440 Hz generator from a Q125 standards module. Although the sine wave from that is noticeably more pure, the artifacts were much the same. So I'm thinking that some of the artifacts are carrier leakage.)

From 1:24 to 1:45, I turn up the Initial Shift again, and then bring in some Up Feedback, and then from there until 2:09 is the Down Feedback. You can hear the effects of these. Be careful when you listen to the latter part; it hits a couple of of the aforementioned ping-pong resonances that cause level spikes.

Next, I ran a sawtooth wave through the frequency shifter, with far more dramatic and impressive results. Shifting the sawtooth wave by small amounts in either directing results in beating and interference patterns as the harmonics are moved off of the integer relationships; the waveform starts to intermodulate with itself. You can hear this in the Fine Shift demonstration from 2:29-2:48, A large shift upwards transforms the sawtooth wave into something distorted-sounding, eventually ending up sounding a bit like it's overdriving a filter with the resonance turned way up. This is in the clip at 2:50-3:10.

Negative frequency shift is even weirder; as the frequency goes through zero, first the fundemental and then the harmonics all go through zero and get inverted, and you can hear them doing it one at a time as the Initial Shift knob is gradually turned more to the left. And the timbre is just strange. This is in the clip at 3:11-3:35. The really interesting bit here is when the source frequency is varied while in the negative-frequency range. The effect is hard to describe; some harmonics go up in frequency, some go down, and the only words I can come up with is that it turns the sound "inside out". That makes no sense, so you just have to hear it, at 3L36-3:46. The up and down feedback controls were very successful at destroying the waveform, creating a bunch of noises that sounded like various forms of exotic radio broadcasts, or perhaps modem noises. Up feedback is demonstrated at 3:54-4:12, and down feedback is at 4:18-4:35.

Interestingly, through most of the things that I did with the sawtooth, there seemed to be a bit of the unaltered input waveform leaking through. I wondered about that, since I didn't hear it with the sine wave. I think it might be this: a sawtooth wave has a portion of the waveform that is basically an impulse -- on the scope it's nearly vertical. Fourier analysis tells us that an impulse or spike is a waveform of indefinite frequency; in theory, a perfectly vertical spike (impossible to achieve with finite bandwidth) contains every possible frequency. For this reason, I think the frequency shifter simply couldn't do anything with that part of the waveform; the math breaks down. The result is that it sounds like an extremely narrow pulse wave, at the pitch of the unaltered input, is riding through the circuit. That would explain why I did not hear it with the sine wave; a sine wave does not have a steep slope anywhere in its waveform.

Some Usage Ideas

So what can you do with the Encore frequency shifter? Here are a few examples. One use is to create phasing/flanging/stereo simulation effects. Connect a mono signal to the input. Center the Initial Shift knob in the deadband and set both of the feedback knobs on zero. Take a stereo out using the up out as one channel and the down out as the other. Or, for a better enhanced effect, run the up out and down out to an external mixer. Mix the up out with some of the dry signal and pan that hard left; then mix the down out with some of the dry signal and pan that hard right.

A frequency shifter is of course the bees' knees at creating bell, chime, and other clangorous timbres. You'll have the best luck with sounds that don't contain a lot of closely spaced harmonics. I actually had good results feeding FM noises into it. Treat with the appropriate envelopes, and you've got the bells of doom. The nice thing about it is that the apparent pitch of the resulting sound is pretty predictable, much more so than if you use a ring modulator to create bell sounds. So you can make it sorta kinda play in scale. I managed to do some things that played more or less in scale over an octave. Precision tuning it won't do, but that isn't what you get a device for. If you want the strange sound but you also want it to play in tune, sample it; the circuits are very stable and will hold a specific timbre while you set it up for sampling.

However, I found that what this unit does best is brutalize sounds. There are a million ways to make it distort a sound -- but in a completely different way from what a clipping or fuzz circuit does. It excels at creating sounds that have a lot of closely spaced overtones with weird quasi-random beating and pulsing going on. And when you get into the frequency inversion regime, the results are indescribably weird.

The sine and cosine carrier outputs can be used usefully as control voltages for various purposes. For example, you can do a simple rotary-speaker effect by feeding each one to the CV input of a VCA. Feed the same audio into both VCAs, and then take their outputs to a stereo mixer panned left and right. The shift controls will control the speed of the apparent rotation, in either direction.

In short, this isn't a pitch shifter. You won't use it to correct clam notes in a track. You will use it to make bizarre and other-worldly sounds. Getting it to behave predictably may be a bit of a struggle. But if all of life was predictable, what fun would that be? The opportunities for serendipitous discovery are huge here. Run stuff through it, turn the knobs, and see what happens. Then, build a patch that uses that. You may surprise yourself.

About Me

Switched-On Bach and Edgar Winter's Frankenstein were my gateways into electronic music. The nature of sound has always fascinated me, and the possibilities of the synthesizer compelled me even before I knew what one was or how it worked. Since then, I've done primitive sampling by splicing cassette tapes, played synth in a bar band, and added funny noises to various art projects. Now, I finally have the studio, the equipment, and the knowledge to make the music I want to make. So what kind of music do I want to make? Read on...